Population level indicators

In risk characterization, step four, the human exposure situation is compared to the toxicity data from animal studies, and often a safety -margin approach is utilized. The safety margin is based on a knowledge of uncertainties and individual variation in sensitivity of animals and humans to the effects of chemical compounds. Usually one assumes that humans are more sensitive than experimental animals to the effects of chemicals. For this reason, a safety margin is often used. This margin contains two factors, differences in biotransformation within a species (human), usually 10, and differences in the sensitivity between species (e.g., rat vs. human), usually also 10. The safety factor which takes into consideration interindividual differences within the human population predominately indicates differences in biotransformation, but sensitivity to effects of chemicals is also taken into consideration (e.g., safety faaor of 4 for biotransformation and 2.5 for sensitivity 4 x 2.5 = 10). For example, if the lowest dose that does not cause any toxicity to rodents, rats, or mice, i.e., the no-ob-servable-adverse-effect level (NOAEL) is 100 mg/kg, this dose is divided by the safety factor of 100. The safe dose level for humans would be then 1 mg/kg. Occasionally, a NOAEL is not found, and one has to use the lowest-observable-adverse-effect level (LOAEL) in safety assessment. In this situation, often an additional un-... 

The relationship between biomarker responses and effects at the population level can be tested in both field experiments and more controlled experiments in mesocosms. It may be possible to define thresholds for biomarker assays performed on indicator species, above which population effects have been shown to occur. Indicator species may be either free living or deployed. The advantage of the latter is... 

Apart from the use of this approach to study the ecotoxicology of neurotoxic pollutants in the field, it also has potential for use during the course of environmental risk assessment. An understanding of the relationship between biomarker responses to neurotoxic compounds and effects at the population level can be gained from both field studies and the use of mesocosms and other model systems. From these it may be possible to define critical thresholds in biomarker responses of indicator species above which population effects begin to appear. In the longer term, this approach... 

General population studies indicate that the activity of ALAD is inhibited at very low PbB levels, with no threshold yet apparent. ALAD activity was inversely correlated with PbB levels over the entire range of 3-34 pg/dL in urban subjects never exposed occupationally (Hemberg and Nikkanen 1970). Other reports have confirmed the correlation and apparent lack of threshold in different age groups and exposure categories (children—Chisolm et al. 1985 children—Roels and Lauwerys 1987 adults—Roels et al. 1976). Inverse correlations between PbB levels and ALAD activity were found in mothers (at delivery) and their newborns (cord blood). PbB levels ranged from approximately 3 to 30 g/dL (Lauwerys et al. 1978). 

The frequency distribution of the TMRP measurements shows for both levels of exposure a bi-modal histogram - in contrary to the log-normal distribution of the control values (Figure 7 ) The bimodal distribution could be due to a reaction at the cellular level indicating a critical number of hits, i.e. peak A is representing the cell population with a TMRP unaffected by low level alpha exposure, while peak B represents cells reacting to alpha hits. 

Indicators of microbial activity in soil represent measurements at the ecosystem level (e.g., processes regulating decomposition of organic residues and nutrient cycling, especially nitrogen, sulfur and phosphorus). Measurements at the community level include bacterial DNA and protein synthesis. Frequency of bacteriophages is a measurement at the population level. 

Responses at the population level may clarify the potential role of chemosignals in the diversification of Liolaemus. The chemical profiles of the precloacal secretions of two populations of L. fabiani indicate that they show some level of differentiation (Escobar et al. 2003). However, populations of L. lemniscatus and L. tenuis did not show clear patterns of recognition of their own population vs. other populations apparently, chemosignals may not play a key role in the speciation process of Liolaemus (unpublished data). 

Reference values indicate the upper margin of the current background exposure of the general population and [are used] to identify subjects with an increased level of exposure (Jakubowski and Trzcinka-Ochocka 2005) compared with the background population level. Those values are derived from data on blood, urine, and other tissues collected from population studies (Ewers et al. 1999). Reference values may be derived differently for susceptible groups if physiologic differences are substantial (for example, children vs adults) (Ewers et al. 1999). 

Homocysteine isn t harmless. Evidence from population studies indicates that high levels of homocysteine in the blood are correlated with heart disease. Folic acid supplements may prevent this problem by ensuring that the homocysteine is rapidly converted to methionine. Similarly, pregnant women generally take folic acid supplements to prevent their babies from being bom with neural tube defects. The mechanism for its action isn t known, but the folic acid may help decrease the level of homocysteine in this case as well. 

In Table III, mating communication disruption has been used as an indicator of the success of pheromone treatment. Male moth catch in (+)-disparlure-baited traps has been normalized against the number of larvae trapped pre-season, thus introducing a measure of relative population into the calculation of comparative effectiveness. The results obtained in 1980 in Maryland illustrate not only the variation of response with dose, but also the dependence of the efficacy of the technique on population levels. 

Channon, D., Cole, M., Cole, L. (2000). A long-term smdy of Rattus norvegicus in the London borough of Enfield using baiting returns as an indicator of sewer population levels. Epidemiol. Infect. 125(2) 441-5. 

The populations are normalized to the most probable level, indicated by parentheses. The estimated errors in Nv" for BaF and BaCl are 5 % for the most highly populated levels for Bal and BaBr the errors are larger, perhaps 10 % (excluding the unknown systematic error in the qv v" for BaBr). 

Keshan County in NE China (Wang and Gao, 2001). During the years of peak KD prevalence (1959-1970), 8,000 cases and up to 3,000 deaths were reported annually (Tan, 1989). The disease occurred in a broad belt from NE to SW China, where subsistence farmers depended on local food supplies. White muscle disease in animals occurred in the same areas. Grain crops in the affected areas contained <0.04 mg Se kg which led to extremely low dietary intakes (10-15 p.g Se d ) in the local population. A very low selenium level in the affected population was indicated by selenium concentrations in hair of <0.12 mg kg (Tan, 1989 Xu and Jiang, 1986 Yang and Xia, 1995). Supplementation with 50 pg Se d prevented the condition but had no effect on those already showing signs of the disease. 

Figure 1 Model of how genotoxic and nongenotoxic contaminant exposure may lead to population-level consequences such as declines and extinction. The gray boxes indicate key steps in the pathways leading to population-level responses of contaminant exposure. Within the boxes, the up arrows suggest increases and the down arrows suggest decreases in the endpoints of toxicant exposure.

When effects of pollutants are seen at the community or ecosystem levels, it may be too late to start reclamation activities, which become inefficient and extremely costly. That is why invertebrates have been effectively used as sources of biomarkers. They can be extrapolated to actual or potential changes at the population level, and considered as predictive tools to assess changes and consequences on community and ecosystem levels. Only some biomarkers identified in invertebrates are highly specific and sensitive, the majority are less specific to chemicals and then indicate the exposure or toxic effects of their mixture. In invertebrates biomarkers can be used as the diagnostic tools of their health or would give the basis for predicting the fate of stressed environment and to start remediation activities. 

Radiation toxicology is a specialized area of toxicology that relates to both characterizing radiation exposure of humans and evaluating the expected health consequences of the radiation exposure. It is a well-researched discipline that provides a wealth of knowledge about both the adverse and beneficial effects of radiation exposure. The indicated research covers the molecular, cellular, systemic, organism, and population levels. 

Physiological and behavioral indicators of impact within a population are the classical means by which the health of populations is assessed. The major drawback has been the extrapolation of these factors based upon the health of an individual organism, attributing the damage to a particular pollutant and extrapolating this to the population level. 

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